Monofilament sutures generally exhibit less tissue drag and causes less tear because they have smoother surfaces than braided multifilament sutures. Monofilament sutures, in general, do not provide the capillarity found in multifilament sutures, which would minimize the spread of wound infection with bacteria and the like. However, since monofilament sutures comprise a single filament there are the following disadvantages: they are less flexible than multifilament sutures; it is more difficult to tie a knot; and the tied knot is more likely to loosen due to inferior knot security.
Particularly, monofilament sutures are less flexible, which results in difficulties in handling and in tying during surgical operations. Moreover, due to the fact that the ears of the tied suture remaining inside the body may irritate adjacent tissues, patients often complain of pain. In addition, even if a marketed monofilament suture is relatively flexible, its knot is easily untied. Therefore, in order to make the knot secure additional throws while tying are required. Such additional throws increase the amount of suture remaining inside the body, and, consequently, increase the irritation caused by the foreign material in the wound. The increase in foreign body, even in the case of an absorbable suture with good biocompatibility, may provoke irritation in adjacent tissue, and thus, increase the probability of inflammation. Furthermore, a patient may feel sensations or stimulation from the knots. The larger the volume of tied knots there is, the more likely it is that undesirable symptoms will present. Van Rijssel E J C, et al., Mechanical performance of square knots and sliding knots in surgery: A comparative study, Am J Obstet Gynecol 1990; 162:93–7, Van Rijssel E J C, et al.; Tissue reaction and surgical knots: the effect of suture size, knot configuration, and knot volume, Obstet Gynecol 1989; 74:64–8; and Trimbos, J. B., Security of various knots commonly used in surgical practice, Obstet Gynecol., 64:274–80, 1984.
In order to overcome on the above disadvantages of a monofilament suture, various methods for improving flexibility of monofilaments have been developed. For example, there is disclosed a process for manufacturing monofilament sutures by modification of a homopolymer (U.S. Pat. No. 5,451,461) or by using a copolymer (Monocryl® suture, a new ultra-pliable absorbable monofilament suture, Biomaterials, v16, 1995, pp 1141–1148). However, the process has limits in improving the flexibility of the suture. Also, even when flexibility is improved, the problem of poor knot security remains. When two or more polymers are combined together, disadvantages of one polymer may be offset by advantages of the others.
U.S. Pat. Nos. 5,626,611; 5,641,501; 6,090,910; and 6,162,537 disclose processes for preparing a suture by using different polymers. However, they are for techniques for controlling the absorption rate when absorbable sutures are degraded in the body. U.S. Pat. Nos. 5,641,501 and 6,090,910 relate to sutures prepared by physically mixing two kinds of polymers. When two polymers are physically mixed and spun into a yarn, the two polymers are not homogeneously distributed over each other, and phases of melted polymers are easily separated therewith. Thus, it is hard to spin the polymers into yarn and it is difficult to make the sutures have homogeneous physical properties.
U.S. Pat. No. 5,626,611 relates to a suture prepared by co-extruding polymers into a sheath/core type, in order to control the absorption rate of the suture. That is, it relates to a method for controlling the absorption rate in accordance with the absorption rate of each polymer used in the sheath or core portion. U.S. Pat. No. 6,162,537 relates to a process for co-extruding a non-absorbable polymer and an absorbable polymer, in order to improve the biologic response of non-absorbable polymers in the body.
As described above, there has been much research in improving the flexibility and strength of sutures and into the techniques for controlling absorption rates. However, research into improving knot security, as one of the important requirements of a suture, has not been enough. Therefore, the present invention provides a suture with excellent knot security and flexibility, which helps overcome the disadvantages of currently marketed monofilament sutures.